cyma reversa vs cymatium

cyma reversa and cymatium both are nouns.

Word	Noun	Adjective	Verb	Adverb
cyma reversa	Yes	No	No	No
cymatium	Yes	No	No	No

As nouns, cymatium is a hypernym of cyma reversa; that is, cymatium is a word with a broader meaning than cyma reversa:

cyma reversa: a molding that (in section) has the shape of an S with the convex part above and the concave part below
cymatium: (architecture) a molding for a cornice; in profile it is shaped like an S (partly concave and partly convex)

Other hypernyms of cyma reversa include cyma.

cyma reversa (noun)	cymatium (noun)
a molding that (in section) has the shape of an S with the convex part above and the concave part below	(architecture) a molding for a cornice; in profile it is shaped like an S (partly concave and partly convex)